#container{
    width: 800px;
    background-color: orange;
    margin: 0 auto;

    overflow: hidden;/* 用于解决高度塌陷 */
}

#container .item{
    width: 200px;
    height: 200px;
    background-color: #0cc;
    float: left;
    margin-right: 20px;
}
#container2 {
    width: 800px;
    height: 300px;
    background-color: cadetblue;
}